As is well known, in a daily production process, such a situation that a rectilinear motion mode needs to be converted into a circular motion mode and even different rotating speeds are required in a circular motion process is often encountered. At this time, the situation needs to be realized by a conversion device. However, most of the existing conversion devices are relatively complicated in structure and inconvenient in speed regulation. For example, a traditional bicycle adopts a manner of pedaling in circles with both feet; since an arm of force is continuously changed between 0 and a maximum value repeatedly and a moment of force is also changed between 0 and a maximum value, physical strength and work time of riders are wasted; and a chain is changed between different chain wheels during speed regulation, so speed changing time is long.